What are the individual privileges required to run a Schema Compare for all objects? (4238510)

If DBO privileges are not allowed to be granted to a user, what are the individual privileges required to run a Schema Compare for all objects?

With 'db_datareader' granted, the following message appears when running the Schema Compare:
'Some objects were not loaded'.

A database user should have db_ddladmin privileges to run Schema Compare.
Or a database user can be created from login and Altered. REFERENCE privileges should be granted to this user.

Example script:
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Create login  username with password = '345' - this login has only public rights
go

use db1
go

create user username from LOGIN username
go

GRANT ALTER, REFERENCES TO username;
Go

use db2
go

create user username from LOGIN username
go

GRANT ALTER, REFERENCES TO username;
go
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